Q: How do you know when compost is finished?

I’ve answered this question in a few, oblique ways across the course of writing this newsletter—but it’s a question I get so much, and with such concern!, from readers, that I wish to dedicate an entire edition.

How do I know when to use my compost?

As with most things compost, there’s no singular or objective answer. There’s no test you can run on your compost that will ding a result saying “I’m finished!” There are, however, a set of questions you can ask when gauging if your compost is ready or not. I find these extremely helpful.

First, what do you see? 

The compost should be dark and crumbly, a lot like soil. Put your hand into your pile and sift it through your fingers. If there are still patches of slime or visible and “undigested” food scraps, your compost isn’t ready yet. Let it keep sitting. If you have bulky twigs or woodchips remaining, though, that’s fine. They can be sifted out before use, or simply applied alongside your compost (which is what I do). What you really want to avoid is putting any undecomposed food directly onto your plants.

What do you smell? 

Compost that is ready to be used smells like fresh, good earth. It’s not an odor you have to strive to perceive. Once my friend Vivian came over and stepped onto my back porch, just as—unbeknownst to her—I opened up my pile for turning. “What smells so amazing?!?” she screamed. The odor of fresh earth had immediately and vibrantly permeated the surrounding air. If you’re gun-shy, though, you can always deploy the “bag test.” Take a handful of your compost from the interior of your pile, moisten it, and seal it within a plastic bag. Leave it there for five days. When you open the bag again, you should smell only fresh earth. If you catch a whiff of ammonia, or anything else unpleasant, that indicates that your pile is not finished decomposing. Honestly, though, I discourage the bag test. It takes forever and I believe in cultivating a direct relationship with the state of your pile. Trust yourself.

What do you feel?

If the pile is still warm to the touch, that means that decomposition is still active, and your compost is not yet ready to use. The temperature test is probably the most reliable and “scientific” indication of whether or not your compost is ready to use, for those of you that like numbers. (Numbers are cool!)

Okay, but then how do I use my compost? 

Well, first relax. It’s very hard to use compost “wrong.” Given that so much stuff, scattered all over the earth, exists in various states of uncontrolled decomposition, all around and on top of all kinds of plants—I would say that the ecosystem is quite adapted. Plants are used to navigating decay in place. That said, you’re likely trying to garden with intentionality, and you want to use your compost in a way that will best support the quick and simple flourishing of your garden. With that in mind, there are three general ways to put your finished compost to use.

Mix it in beforehand

Your finished compost can be very valuable for helping prepare a garden bed before you’ve planted anything. Just mix three to four inches of compost into the top soil of your garden bed (use a rake a shovel or, if you’re like me, your bare hands) and leave it be. I generally recommend doing this at least a few months in advance of planting anything. This gives time for the microbes in your compost to become integrated into your soil, and it also provides a cushion of time for any possibly-still-active decomposition to complete.

You can also mix compost into potting soil before planting an indoor plant. Use a 1:4 ratio of compost to potting soil.

Use it when you’re planting!

Throw a handful of finished compost into a freshly dug hole, before setting in your plant. Or mix your seeds with compost before broadcasting them across a lawn or garden bed. Don’t overthink it. Just do it!

Put it on top

Compost can also provide valuable top dressing for a mature garden bed or your houseplants. Sprinkle it directly onto the soil as often as you like. (If you notice your plants look droopy or “sad,” that might be a sign they need a little boost of micronutrients, which compost can provide.) Sprinkle and see how they look the next day. You can also do more than sprinkle, and add a full ~inch of compost to the top of your garden soil or potted houseplants. You don’t have to do this very frequently in order to see the results. Once every few years in your garden, twice a year for your houseplants. The compost will provide a slow release of micro- and macro-nutrients, which should keep your plants pretty happy for a long while. Isn’t that nice?

There’s really no wrong time to add finished compost to your plants, though. And there’s really no such thing as using too much. Feel free to sprinkle your compost amongst your plants liberally and/or weekly. Or save it up for a big garden bed preparation session in the fall.

One other option, and this is my favorite option, is to give your compost away. Not all those who compost also garden, and we may not have great use for our finished product. Local schools, community gardens, gardeners who are neighbors, friends, and others will all possibly appreciate a big ol’ bag of your fresh and wonderful compost.

Love,

Cass

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